The invention relates to an assembly for sensing and/or monitoring the level of a material in a vessel with the aid of a level sensor comprising a mechanical vibrator system and an electromechanical transducer system, the level sensor being applied to the vessel so that the mechanical vibrator system comes into contact with the material when the material has attained a predetermined level, and wherein the transducer system is connected to a low-frequency emitter circuit configured so that it induces low-frequency vibration in the mechanical vibrator system, the assembly further comprising a low-frequency detector circuit furnishing from evaluation of the frequency and/or amplitude of an electrical signal furnished by the transducer system an output signal indicating whether the mechanical vibrator system has come into contact with the material or not.
An assembly of this kind is known, for example, from DE 33 36 991 A1. In this known assembly the mechanical vibrator system consists of two vibrating rods secured juxtaposed spaced away from each other to a diaphragm, the rim of which is connected to a grommet. The electromechanical transducer system comprises an exciting transducer and a receiving transducer. When an alternating voltage is applied to the exciting transducer it acts on the side of the diaphragm facing away from the vibrating rods so that the vibrating rods are caused to vibrate opposingly transversely to their longitudinal direction, and the receiving transducer converts the vibrations of the mechanical vibrator system into an electrical alternating voltage. The two transducers are connected to an amplifier in a self-exciting circuit so that vibration of the mechanical vibrator system is excited at its natural resonance frequency. The natural resonance frequency of the mechanical vibrator system depends on whether the vibrating rods are vibrating in air or are immersed in the material in the vessel; the natural resonance frequency in the immersed condition being lower than when vibrating in air, but in any case is below 1000 Hz in the low-frequency range. The low-frequency detector circuit compares the frequency of the electrical alternating voltage existing in the self-exciting circuit--this frequency being the same as the natural resonance frequency of the mechanical vibrator system--to a threshold value, and outputs an output signal indicating whether this frequency is above or below the threshold value.
An assembly known from DE 32 15 040 C2 for sensing and/or monitoring a predetermined level of a material in a vessel includes a mechanical vibrator system formed by a tubular hollow body in the cavity of which a transverse member is arranged secured by two opposing fastener points to the inner wall of the cavity. The transducer system, containing an exciting transducer and a receiving transducer, cooperates with the transverse member which can be caused to vibrate radially which is transmitted to the wall of the tubular hollow body. The natural resonance frequency of this mechanical vibrator system is in the range 20 to 30 kHz. The vibration of the outer wall of the cylindrical hollow body is damped by coming into contact with the material, the amplitude of the alternating voltage output by the exciting transducer changing accordingly. The exciting circuit compares the amplitude of this alternating voltage to a threshold value to indicate whether the tubular hollow body has come into contact with the material or not.
Known on the other hand are assemblies for sensing and/or monitoring the level of a material in a vessel which contain no mechanical vibrator system vibrating at its natural resonance frequency, they instead sensing the presence of a material at a predetermined level with the aid of elastic waves, the frequency of which is in the ultrasonic frequency range. Thus, EP 0 409 732 B1 or the corresponding DE 690 08 955 T2 describes a detector for indicating a liquid with a vertically arranged waveguide for elastic waves, the lower end of which is located level with the material level to be monitored. The waveguide has two cylindrical portions of which the upper portion is fully cylindrical and the lower portion may be fully cylindrical or tubular. A transducer mounted on the upper end of the waveguide emits pulsed elastic waves which run through the waveguide to the lower end. The frequency of the elastic waves is of the order of 75 to 125 kHz. When the lower end of the waveguide is not in contact with the liquid the emitted wave train is reflected at the end of the waveguide so that it is returned as an echo of larger amplitude to the transducer by which it is converted into an electrical detector signal of correspondingly high amplitude, whereas when the lower end of the waveguide is immersed in the liquid the wave train emitted by the transducer is absorbed by the liquid, resulting in a considerable attenuation of the reflected echo. By comparing the amplitude of the electrical detection signal to various threshold values it can thus be likewise established where the level is located within a specific section of the lower portion of the waveguide.
Available on the market are thus low-frequency and high-frequency systems for sensing and/or monitoring the level in a vessel by means of mechanical vibrations. Typical frequencies of the low-frequency sensing systems are in the audible sound range below 1000 Hz whilst typical frequencies of the high-frequency sensing systems are in the ultrasonic range above 15 kHz.
Each of these principles has its pros and cons depending on the material to be sensed and the prevailing ambient conditions. For instance, high-frequency sensing systems cause problems where gassy materials, such as carbonated beverages, are concerned since bubbles a t the mechanical vibrator system result in heavy smearage of the sensing signal. This problem can only be resolved by a complicated analysis of multiple reflections, whereas low-frequency sensing systems have no trouble working with gassy materials. The way in which the systems react to deposits having formed is also different. Where a soft deposit is concerned low-frequency sensing systems work unreliably, whereas high-frequency sensing systems are reliable, their response being the opposite in the case of hard deposits.
In conclusion, low-frequency sensing systems are ruined by low-frequency alien vibrations and high-frequency sensing systems by the high-frequency alien vibrations, whilst the one type of sensing system is not detrimented by the alien vibrations of the other frequency range in each case.